The present invention relates to an apparatus for positioning packing boxes, in particular for covering boxes with a pre-glued covering sheet.
There are prior art so called “covering machines” for joining an erected box and a covering sheet to which glue was previously applied.
Such machines have a movement system with which the box is conveyed to a unit which applies the glued sheet to the walls of the box to complete box covering.
The boxes are brought to the covering unit already joined to the respective covering sheet. In this situation, the box rests on the pre-glued inner surface of the sheet at a central portion of the covering sheet, with the respective opening facing upwards.
For this purpose covering machines have a centering or “aiming” station, positioned along a conveyor line consisting of a belt with suction which holds the sheet and conveys it from the gluing roller to the covering unit, in which the box is centered on the pre-glued sheet.
Box centering is made possible by sensors which optically detect the position of the sheet on the conveyor belt and consequently control box positioning at the moment it is joined to the sheet.
To identify the position of the sheet on the conveyor belt, the optical sensors are aimed at the sheet at specific points where they are able to detect a color contrast between the sheet and the belt below, thus allowing precise calculation of the position for box centering on the sheet.
Operation of the prior art systems is satisfactory when the color difference between the sheet and the belt makes recognition of the position of the sheet certain.
Otherwise, for example if dark sheets are conveyed by a belt which is also dark, it is possible and often the case that there are positioning errors and consequently finished box production rejects.
To overcome this disadvantage, it is common practice in the sector to apply longitudinal adhesive strips at the sensor reading areas, the adhesive strips being colored with a color that contrasts with the sheet color.
However, this solution has a number of disadvantages linked to the strips wearing and the uncertainties that this involves in detecting the position of the sheet.
Moreover, to allow the reading of sheets having different dimensions, relatively wide strips would have to be used (referring to the transversal dimension of the belt), which allow reading even for large sheets but which at the same time, due to their width, would reduce the capacity for suction of the sheet on the belt and therefore the stability of its position.
The disadvantages referred to are particularly felt when one wants to change the color and size of the sheet rapidly without having to interrupt production.